There are certain contexts in which power or current control and current unbalance compensation are desirable to mitigate the inefficiencies and potential damage that can result from fluctuating power or current demands and unbalances. For example, in the case of alternating current (AC) Electric Arc Furnace (EAF) loads, electric arcs are created among a number of electrodes and the material in the furnace in order to heat, melt or smelt metals, ore or other materials. These arcs behave as time varying and mostly resistive impedances. Variations in arc resistance cause related variations in the power and current drawn by the furnace. The relationships between arc resistance, power and current are shown graphically in FIG. 1A.
In FIG. 1A, Curve A shows variations in arc resistance from low values (a short circuit) to high values (a loss of arc). An exemplary operating resistance and current point is marked with a dot on Curve A. Curve B of FIG. 1A shows the power drawn by the arc versus the arc current for the corresponding arc resistance variations. An exemplary power and current operating point is also marked with a dot on Curve B. The power or current fluctuations resulting from arc resistance variations affect the power supply system and cause frequency and voltage disturbances, which may negatively impact the operation of the power supply, and other furnace loads connected to the same power supply. For example, a sudden and large power upswing in the arc furnace may trigger a load shedding relay or a generator under-frequency relay to trip, resulting in a total loss of power to the arc furnace and leading to potentially substantial economic loss from the loss of productivity. Additionally, the voltage fluctuations at certain frequencies may cause light flicker.
Another problem also arises when an AC EAF is connected as a three-phase load to a three-phase power supply. As the arc resistances in the furnace may not always be equal amongst the three phases, the current may be unbalanced between the phases. Unbalanced currents can in turn cause voltage unbalances, which can affect the operation of other loads, such as electric motors, for example, connected to the same power supply. If the current unbalance exceeds the unbalanced current limit of the supply system generators, the unbalance may cause relays within the power system to trip, resulting in a loss of power to the furnace.
Traditionally, EAF operations have been controlled by electrode positioning systems to operate at a desired set-point of power, current or impedance. The electrode positioning systems generally rely on moving mechanical parts and typically lack the speed and flexibility to respond adequately to fast resistance changes.
To reduce the amount of resistance fluctuations, one or more series reactors may be added to the furnace power supply. The series reactors force the EAF to operate at a lower power factor and therefore allow more stable arcing. However, the reactors alone may be inadequate to obtain the desired level of power stability. Additionally, reactors alone are not an effective means of current unbalance reduction. This is because their reactance values may not change as quickly as the speed at which the arc resistance changes.
Some attempts have been made to temper the effect of power fluctuations in electric arc furnace installations. For example, U.S. Pat. No. 6,603,795 to Ma et al., the entire contents of which is hereby incorporated by reference, describes a system for stabilizing the power consumption in an electric arc furnace by using variable reactor control and electrode height regulation to reduce active power fluctuations. The system monitors the operating characteristics of the furnace, such as the electrode impedance, and makes corresponding adjustments to the variable reactance. The reactance in the circuit may be controlled by adjusting the firing angle of a set of thyristors that couple a reactor to the circuit.
FIG. 2 shows a diagram of a simplified circuit 10 of an electric arc furnace in accordance with Ma et al. The circuit 10 shows a line voltage 12, arc impedance 14, a fixed circuit reactance 16, and a variable reactance 18. The arc impedance 14 includes an arc reactance Xarc and an arc resistance Rarc. The fixed circuit reactance 16 may include the reactance of the furnace transformer and any power cables, conductors, and bus work between the supply system and the electrode, where that reactance can be considered constant as compared to the arc impedance 14.
The power control system described in Ma et al. varies the reactive impedance of the electrodes of an electric arc furnace and the power supply line in response to measured characteristics of the furnace. The system monitors the voltage and current drawn by an electrode in the electrode arc furnace and determines the electrode impedance. Based upon the electrode impedance, the power control system adjusts the reactive impedance to minimize power consumption fluctuations of the arc furnace, as seen by the power supply. It does this by adjusting a variable reactance. The response time associated with this control system is in the order of about one electrical cycle, providing for a relatively fast response.
Ma et al. also describe an electrode position controller that controls an electrode positioning system to adjust the electrode height based upon measured characteristics of the electrode. For example, the electrode position controller may monitor the electrode impedance by monitoring the voltage and current characteristics for the furnace and may regulate the electrode height to minimize power fluctuations due to changes in the electrode impedance. The response time of this control system is relatively slow, being in the order of several seconds.
The system described in U.S. Pat. No. 6,603,795 to Ma et al. is generally able to minimize the power swings and maintain a desired set point for a single furnace. However, the system has a limited capacity to minimize larger power dips under a particular threshold. Moreover, the system is not designed to minimize load unbalances amongst three electrical phases.
FIG. 1B illustrates the limitation of the system described in U.S. Pat. No. 6,603,795 to Ma et al. in minimizing larger power dips under a particular threshold. Curve C in FIG. 1B shows the required variable reactance in order to compensate for the variation in the load resistance in meeting the specific power set-point. Curve D shows the amount of variable reactance that is obtained, taking into account the practical size limits of the variable reactor. The required reactance and current at the desired operating points are also marked. Curve E shows the power drawn by the arc furnace versus the current drawn for the corresponding arc resistance variations when the variable reactance of Curve C is inserted in the circuit. The resulting operating point power and current are also marked.
As illustrated in FIG. 1B, if the electrode current I drops below a critical value Icritical (for example, when the arc is extinguished under one electrode), the circuit 10 will be unable to maintain the power at a fixed level and the power will drop below the power set point. The critical value Icritical coincides with the variable reactance 18 being reduced to its minimum value. There may also be a maximum variable reactance setting that limits the ability of the circuit 10 to maintain the power at the set point if the current rises above a maximum current value, Imax.
It is desired to address or ameliorate one or more of the shortcomings or disadvantages associated with previous control systems and methods for controlling power and/or current in electrical furnaces, or to at least provide a useful alternative thereto.